Coalescers are used extensively in oil and gas production to help separate oil and water mixtures. In many production situations a stream of production fluid contains a mixture in the form of an emulsion. The emulsion is a suspension of very small droplets of one fluid phase (for example water) carried in another fluid phase (for example oil). Coalescers are used to increase the size of the droplets. This helps to break down the emulsion and larger droplets separate much more easily (for example under gravity).
An important development in the use of coalescers was the advent of the Compact Electrostatic Coalescer (CEC®; a registered trademark owned by the applicant/assignee of the present application). This device, which is described in European Patent No. 1,082,168 B1, reduced substantially the size requirement of electrostatic coalescers that could be used effectively in separation plants, which is of particular benefit for production installations where space is at a premium. However, a further benefit of the CEC® is its ability to handle a very wide range of oil-to-water ratios in the mixture, and particularly mixtures having a high proportion of water (high water cut). Prior designs of electrostatic coalescers were not able to handle high water cuts due to problems arising from conduction in the water (effectively short-circuiting of the electrodes). In the CEC®, the energised electrodes are fully encapsulated in insulation, which enables an intense electrostatic field to be applied to the emulsion.
Another feature of the CEC® is the creation of narrow flow gaps for the fluid flowing past the electrodes. These help to ensure that there are no recirculation paths for the emulsion, which would reduce the effectiveness of the coalescing. The narrow gaps also help to create a turbulent or transitional flow regime, avoiding laminar flow which would also reduce the effectiveness.
A particularly suitable design of CEC®, as described in EP 1,082,168, includes a series of nested annular cylindrical electrodes of progressively increasing diameter arranged with narrow annular flow gaps between them. The electrodes are situated inside a cylindrical vessel with a fluid inlet at one end and a fluid outlet at the other end. The preferred orientation is with a vertical cylinder axis, such that the fluid inlet is above the electrodes and fluid flows vertically downwards towards the fluid outlet. This is because coalesced water droplets begin to settle under gravity, and a vertical orientation prevents a water layer forming next to an electrode. Such a water layer changes, and could adversely affect, the electrostatic field. However, effective coalescing of droplets in an emulsion requires a certain residence time of the fluid in the electrostatic field. This residence time results in a minimum required length of electrode, which in turn dictates a minimum overall height of the coalescer.
The known CEC® such as the ones disclosed by EP 1,082,168, generally comprise a cylindrical vessel mounted vertically and having an inlet for a fluid mixture, such as an oil and water emulsion. The inlet is situated near the top of the vessel. Inside the vessel are one or more vertically aligned electrodes, having narrow flow gaps such that the fluid mixture flows in the indicated flow direction past the electrode surfaces. The electrodes are fully insulated and are connected to a high voltage supply, allowing an intense electrostatic field to be generated in the narrow gaps.
It will be appreciated that one way to generate an intense electrostatic field in the narrow gap, is for an electrode at one side of the gap to be energised by the high voltage, while the surface at the other side of the gap is held at ground potential. This means that, in a particular arrangement where there are narrow gaps between a series of electrode structures, the high voltage energises each alternate electrode structure, while the other alternating electrode structures in between are held at ground potential. In such cases, only the energised electrodes need to be insulated.
As the fluid mixture passes through the narrow gaps, the effect of the electrostatic field is to coalesce droplets of one phase (e.g. water). The larger coalesced droplets are then more easily separated (for example in a settling vessel located downstream of the CEC®). This effect is enhanced if the fluid flow is in a turbulent (or at least transitional) flow regime (Reynolds No. >2000, typically in the range 2000-8000).
After the fluid has passed the electrodes, it enters a flow calming section at the bottom of the vessel before passing out through an outlet. The flow calming section helps to prevent re-mixing or breaking up of the coalesced droplets.
The height of the known CEC® is determined largely by the height (i.e. vertical extension; length) of the electrodes, which is determined by the required residence time of the fluid in the electrostatic field, while maintaining a flow velocity in order to ensure that the flow is in a turbulent or transitional regime (i.e. not in a laminar regime). In a typical known CEC® the electrodes are 2 m in height, making the overall height of the CEC® substantially more than this.
There are many situations where a compact electrostatic coalescer is used in association with other process equipment, such as settling or separation vessels. Space is often at a premium, especially on off-shore production platforms, and in some circumstances the height available may be restricted. It is therefore a need for an electrostatic coalescer which is reduced in size, while maintaining the process conditions and capabilities.